This invention relates to integrated optic waveguide couplers, i.e. those in which power in one waveguide is coupled to an identical adjacent waveguide over an interaction length throughout which the waveguides lie parallel and are in contact with each other, the degree of coupling depending upon the interaction length and varying for different wavelengths. Some power transfer will also take place between waveguides which are sufficiently closely spaced, but not necessarily in contact, this also being a function of wavelength and also on the degree of separation. Accordingly power transfer in the interaction length will be supplemented to some extent by closely spaced regions of waveguides adjoining the interaction length, and which will hereinafter be called the additional interaction length.
The minimum length over which coupling takes place for full transfer of power at a given wavelength is known generally as the coupling length.
Now it can be shown that the amount of power transfer for a given wavelength depends also on the curvature of contacting or closely spaced waveguide sections, and this characteristic is made use of in the present invention. The invention is particularly useful in passive star couplers, for example 2.times.N or N.times.N star couplers, in which power from each input waveguide is distributed equally among N output waveguides. Such star couplers are useful in optical communications networks.
It is advantageous in optical communications networks which involve optical signals of two or more different wavelengths for the star couplers to be wavelength insensitive over the range of wavelengths for which the network is designed.